Multiple station wafer processing systems are an integral part of the manufacture of integrated circuits and other wafer based products. These systems typically have multiple coating stations which allow a variety of materials to be deposited on the wafer surface. Patterning and etching stations may be interspersed between the coating stations.
A single system with multiple processing stations offers a number of benefits over the use of separate processing systems for each stage of production. First, wafer fabrication in a multiple station system is much faster than that obtainable using individual processing systems since the wafers need not be pretreated between each station and the overall system can be easily automated. Second, a superior product can be fabricated using such a system since the wafers do not become contaminated between production steps. Contamination results not just from airborne contaminants, but also from such sources as water vapor. Therefore it is critical to keep the wafers in an evacuated environment throughout as much of the process as possible. In a multiple station system even during wafer transfer the wafers are never exposed to pressures greater than approximately 5.times.10.sup.-8 Torr.
Multiple station wafer handling systems typically use robotic transfer arms to move wafers from station to station. These transfer arms must be calibrated so that they place the wafers in the correct position within each processing station. If a wafer is not properly placed within a station it may be improperly treated, thus leading to a defective part. Furthermore, this defect may not be readily apparent and thus substantial and costly processing and testing may be completed before the defect is even identified. To further exacerbate the problem, once the transfer arm is out of calibration tens or hundreds of wafers may be processed before the defect is even noticed thus leading to even greater financial loss.
Once it is determined that the transfer arm must be calibrated, the entire handling system must be brought up to atmospheric pressure and the system taken apart to a sufficient degree to enable a technician to calibrate the arm. The arm is calibrated by moving it to each processing station and visually inspecting its location. Based upon this visual inspection, the arm is adjusted and the inspection process is repeated. This procedure continues until the system user is satisfied with the arm's calibration, at which time the system is reassembled and evacuated. Assuming that only minor transfer arm adjustment and calibration are required, this procedure typically requires approximately 16-20 hours. Furthermore the final calibration is not very accurate since it is based solely on a visual review of the placement of the transfer arm within each processing station.
Wafers are often selected for processing on the basis of their crystallographic orientation. This orientation is typically noted on each individual wafer through the placement of a notch or a flat along the wafer's circumference. Therefore it is often important that the wafer handling system have some means of identifying the orientation of each wafer as well as maintaining the proper alignment of the wafers throughout processing.
Presently wafer orientation is determined using a costly laser system. The laser scans each wafer to determine its orientation, and then the wafer is rotated to achieve the desired alignment of the wafer's orientation with respect to the processing system. Once the wafer's orientation is properly aligned, the user must simply assume that the wafer does not become misaligned during subsequent handling and processing. A wafer which becomes misaligned during processing may not meet the required operational specifications, thus leading to its rejection.
From the foregoing, it is apparent that a method of calibrating the transfer arm in a multiple station processing system without breaking system vacuum or incurring a long system downtime is desirable. Furthermore, an easy method of aligning and maintaining the alignment of the wafers during processing is also desired.